Factor IXa is a plasma serine protease involved in the regulation of blood coagulation. While blood coagulation is a necessary and important part of the regulation of an organism's homeostasis, abnormal blood coagulation can also have deleterious effects. For instance, thrombosis is the formation or presence of a blood clot inside a blood vessel or cavity of the heart. Such a blood clot can lodge in a blood vessel blocking circulation and inducing a heart attack or stroke. Thromboembolic disorders are the largest cause of mortality and disability in the industrialized world.
Blood clotting is a process of control of the blood stream essential for the survival of mammals. The process of clotting, as shown in the Patent Document 1, and the subsequent dissolution of the clot after wound healing has taken place commences after vascular damage and can be divided into four phases:
1. The phase of vasoconstriction or vasocontraction: By means of this the blood loss in the damaged area is decreased.
2. The next phase is platelet activation by thrombin. The platelets attach to the site of the vessel wall damage and form a platelet aggregate. The protein fibrinogen is responsible here for the crosslinkage of the platelets by means of appropriate surface receptors. Platelets also bind to exposed collagen of the extracellular matrix of the damaged vessel wall and are activated by this means. After activation of the platelets, a number of messenger substances are secreted, which induce the activation of further platelets. At the same time, a membrane lipid, phosphatidylserine, is transported from the inside of the membrane of the platelets to the outside, on which complexes of clotting factors can accumulate. The platelets accelerate blood clotting by means of this mechanism.3. The formation of these clotting complexes leads to the massive formation of thrombin, which converts soluble fibrinogen to fibrin by cleavage of two small peptides. Fibrin monomers spontaneously form threadlike strands, from which, after crosslinkage by clotting factor XIII, a stable protein network forms. The initially even looser platelet aggregate is stabilized by this fibrin network; platelet aggregates and fibrin network are the two essential constituents of a thrombus.4. After wound healing, the thrombus is dissolved by the action of the key enzyme of the endogenous fibrinolysis system, plasmin.
Two alternative pathways can lead to the formation of a fibrin clot, the intrinsic and the extrinsic pathway. These pathways are initiated by different mechanisms, but in the later phase they converge to give a common final path of the clotting cascade. In this final path of clotting, clotting factor X is activated. The activated factor X is responsible for the formation of thrombin from the inactive precursor prothrombin circulating in the blood. The formation of a thrombus on the bottom of a vessel wall abnormality without a wound is the result of the intrinsic pathway. Fibrin clot formation as a response to tissue damage or an injury is the result of the extrinsic pathway. Both pathways comprise a relatively large number of proteins, which are known as clotting factors.
The intrinsic pathway requires the clotting factors V, VIII, IX, X, XI and XII and also prekallikrein, high molecular weight kininogen, calcium ions and phospholipids from platelets.
The intrinsic pathway is initiated when prekallikrein, high molecular weight kininogen factor XI and XII bind to a negatively charged surface. This point in time is designated as the contact phase. Exposure to vessel wall collagen is the primary stimulus of the contact phase. The result of the processes of the contact phase is the conversion of prekallikrein to kallikrein, which in turn activates factor XII. Factor XIIa hydrolyzes further prekallikrein to kallikrein, such that activation is the result. With increasing activation of factor XII, activation of factor XI occurs, which leads to a release of bradykinin, a vasodilator. As a result, the ending of the initial phase of vasoconstriction occurs. Bradykinin is formed from high molecular weight kininogen. In the presence of Ca2+ ions, factor XIa activates factor IX. Factor IX is a proenzyme, which contains vitamin K-dependent, γ-carboxyglutamic acid (GLA) residues. The serine protease activity becomes noticeable after binding of Ca2+ to these GLA residues. A number of the serine proteases of the blood clotting cascade (factors II, VII, IX and X) contain such vitamin K-dependent GLA residues. Factor IXa cleaves factor X and leads to activation to factor Xa. The prerequisite for the formation of factor IXa is the formation of a tenase complex from Ca2+ and the factors VIIIa, IXa and X on the surface of activated platelets. One of the reactions of activated platelets is the presentation of phosphatidylserine and phosphatidylinositol along the surfaces. The exposure of these phospholipids first makes the formation of the tenase complex possible. Factor VIII in this process has the function of a receptor for the factors IXa and X. Factor VIII is therefore a cofactor in the clotting cascade. The activation of factor VIII with formation of factor VIIIa, the actual receptor, needs only a minimal amount of thrombin. With increase in the concentration of thrombin, factor VIIIa is finally cleaved further and inactivated by thrombin. This dual activity of thrombin in relation to factor VIII leads to a self-restriction of tenase complex formation and thus to a limitation of blood clotting.
The extrinsic pathway requires a tissue factor (TF) and clotting factors V, VII, VIII, IX and X. In the case of a vessel injury, the tissue factor (TF) accumulates with the clotting factor VII and the latter is activated. The complex of TF and clotting factor VII has two substrates, clotting factors X and IX.
Clotting factor IX can be activated by means of the intrinsic pathway and the extrinsic pathway. The activation of factor IXa is thus a central point of intersection between the two pathways of activation of clotting.
Factor IXa has an important role in blood clotting. Defects in factor IXa lead to hemophilia B, while increased concentrations of factor IXa in the blood lead to a significantly increased risk of thrombosis formation (Weltermann A, et al., J Thromb Haemost. 2003; 1: 28-32). The regulation of factor IXa activity can reduce thrombus formation in animal models (Feuerstein G Z, et al., Thromb Haemost. 1999; 82: 1443-1445).
Recently, compounds having a Factor IXa antagonism are being studied. Known compounds each having an amide bond are disclosed in, for example, PCT Publication No. 08/031,508 pamphlet (Patent Document 1), PCT Publication No. 08/031,509 pamphlet (Patent Document 2). However, these patent documents do not disclose cyclic morpholinone derivatives.
In the development of pharmaceuticals, it is required to satisfy strict criteria for not only target pharmacological activity but also absorption, distribution, metabolism, excretion, and the like. With respect to drug interactions, desensitization or tolerance, digestive absorption in oral administration, the rate of transfer to a small intestine, the rate of absorption and first-pass effect, an organ barrier, protein binding, induction of a drug-metabolizing enzyme, an excretion pathway and body clearance, a method of administration (an application site, a method, and purpose), and the like, various agenda are required. However, a drug that satisfies these requirements is seldom discovered.
These comprehensive problems in drug development might also exist for Factor IXa antagonists, and Factor IXa antagonists have not yet been released onto the market. More specifically, known compounds having a Factor IXa antagonism may also include problems in terms of usefulness and safety. For example, these compounds may have low absorption, and oral administration of these compounds may be difficult; these compounds also may exhibit inhibitory activity of the human ether-a-go-go related gene (hERG) channel, which may cause arrhythmia, and pharmacokinetics of these compounds might not satisfactory.
Accordingly, a compound in which these problems are solved and which has high activity has been desired.